The invention relates to a circuit arrangement for a detector for ionizing radiation exhibiting a plurality of measuring fields, in which means for processing the output signals are connected to the output side of each measuring field.
Circuit arrangements of this type are employed in automatic x-ray exposure timers. In such instances, each measuring field delivers a signal which is a measure of the corresponding dose rate of the radiation impinging thereon. By integrating this signal, it is possible to obtain a signal corresponding to the x-ray dose, and to use this signal in order to switch off the radiation.
On the basis of the varying transparency of medical subjects it is necessary, in an automatic radiographic exposure timer of the type described, to ascertain the mean value of the dose over a greater area which is important for the diagnosis. This may proceed through a suitable selection of the radiation-sensitive measuring surface of the detector. Specifically, the radiation-sensitive measuring surface can be determined by a suitable switching on of measuring fields. However, during examinations with contrast agents, for example during the examination of the stomach-intestine passage, it is possible for a greater area of the measuring surface to be covered by the contrast agent, which leads to incorrect exposures by the automatic exposure timer because, in this instance, the radiation dose which is integrated over the measuring surface of the detector in a specific period of time can be greatly divergent from the dose which results when such strong contrasts in the X-ray image are not present.